PROJECT SUMMARY: The matching of blood flow and oxygen delivery to tissue oxygen demand is one of the most fundamental physiological processes. Recent evidence indicates that the red blood cell can act as a ?sensor? and releases ATP during mismatches in oxygen demand and delivery, and this ATP can evoke vasodilation and improve local blood flow under such conditions via binding to purinergic (P2) receptors on the endothelium. In addition to the direct vasodilatory effect, we have recently demonstrated that ATP is also capable of inhibiting sympathetic vasoconstriction (?sympatholytic?), which could further aid in blood flow and oxygen distribution. Aging is associated with impaired blood flow and oxygen delivery during conditions of hemoglobin deoxygenation (e.g. hypoxia, exercise), and we have recently demonstrated that this is associated with a lack of increase in circulating ATP due impaired red blood cell ATP release. Currently, the major critical barrier to our mechanistic understanding of how ATP controls vascular tone in humans is the lack of an effective inhibitor of ATP, P2 receptor-mediated vasodilation. Our preliminary data indicates that local intra- arterial infusions of pyridoxine hydrochloride significantly inhibits ATP-mediated vasodilation in young adults. Thus, the overall goal of this exploratory research program is to directly test the hypothesis that pyridoxine hydrochloride is an effective antagonist that inhibits ATP, P2 receptor-mediated vasodilation in humans in vivo, and to directly determine the role of intravascular ATP in the control of blood flow and oxygen delivery in young and older adults during various physiological stimuli. To test our hypotheses we will address the following specific aims. In Specific Aim 1 we will determine whether the forearm vasodilator responses to local intra- arterial administration of ATP are inhibited by local infusions of pyridoxine hydrochloride. We will also determine the specificity of this inhibition to intravascular ATP by administering various endothelium-dependent and -independent agonists, as well as other purines, to rigorously address this aim. In Specific Aim 2 we will determine whether the forearm vasodilator responses to graded systemic hypoxia and graded rhythmic handgrip exercise are reduced after inhibition of P2 receptors in young and older adults. The methods employed to address these aims are state-of-the-art and involve local (intra-arterial) administration of various study drugs at rest and during systemic hypoxia and exercise, and measurements of forearm arterial and venous plasma ATP concentrations in young and older healthy humans. The findings from the proposed studies will establish the efficacy of pyridoxine hydrochloride as a pharmacological inhibitor of P2 receptors in vivo, and will provide the first data regarding the mechanistic role of ATP in vascular control during physiological stress in both young and older adults. This could serve as the impetus for future studies designed to improve circulating ATP and/or P2 receptor signaling in various patient populations suffering from exercise intolerance or tissue ischemia due to impaired local regulation of blood flow and oxygen delivery.